Sansui AU-919 wake Up!

[samjmann]

I looked at this beast of an integrated amp a few years back, I was never really happy with it as it had further issues… A friend of mine owns it and it was agreed to put to one side till I had more time. The time came a few months ago.

It was fundamentally working, but once one output stage oscillated at HF for no apparent reason. The output transistors for this amp are no longer available and substitutes are a different mount. Fortunately there is a excellent post by LeeStereo on the AudioKarma forum that details a re-cap and details of the so called black-flag caps in the output stage. Failure of these small value 3pf to about 33pF polystyrene caps allows the output stage to oscillate at HF then go bang. Farnell did NPO/C0G replacements. See photos. The picture shows the output block removed and sat on the underside of the amp. This way it was reasonably easy to work on.

All the small electroytics on the output stages were changed, but none measured significantly out. I had already done work on the sub supplies before, so this time it needed the MM and MC preamps looking at and the flat-amp board as well.

With the output stages complete and the flat amp boards done, I could run it from a CD player. It sounded great. But, when on PU all was not well. The MM preamp was oscillating at over 10Mhz. This was enough to put the output stage into protect. How I didn’t damage the output transistors I don’t know. Both channels had this fault. It was the black flag caps that were open in the preamp. The pictures of the scope shows the midpoint of the preamp in oscillation.

I replaced more aged caps in the power supply, however only four were significantly low in value.

I could at last run it safely. I did a set up of the midpoint on each MM pre-amp, both flat amps and of course the output stage. This got rid of the DC offsets that caused the function select switching to click between functions. The switches were not noisey.

Final test in my lounge… It sounds absolutely beautiful. Staggering bottom end and totally clear HF. I’ve never heard my speakers sound as good. I was upset to have to let it go. I honestly wish it was mine!

It had stood for years in damp conditions, there was evidence rust on the chassis. The owner has had the top cover powder coated.

The amp was produced around 1978, it now has quite a following. I can see and hear why.

I will attach more pics on the next post

SJM

[samjmann]

Further pictures..

[Phil G4SPZ]

Well done, Sam! Good fault-finding there. Unusual for such small value caps to fail, and to cause such dramatic and potentially destructive faults, but after nearly 45 years I guess we should expect these problems.

[Uncle Bulgaria]

Very impressive. It looks like quite a job to get in, let alone find the faults. It's good that a nicely drawn manual is available. I hope your friend is suitably delighted!

[Ted Kendall]

Black flags! Hate 'em. I had a Kenwood L-07 preamp on the bench which had been abandoned because it would do nothing but oscillate with a side order of squegging. Every black flag had failed. Once they had been replaced, the thing behaved.

[samjmann]

To be honest I spent hours on this amp. When I first heard the hum on the phono preamp I thought at first it was just poor smoothing. The penny dropped when it went once into protect, I concluded that the hum I had heard was the main smoothing complaining about the HF that was being sent to it from the phono stage!! The scope reavealed the waveform at the 'midpoint output of the preamp. I was very lucky not to have destroyed the o/p transistors.

There was also the dreaded glue here and there. In one place it was on some terminals of the Ht caps, it had blackned with result of volatage on it.

Everything about the design of this amp was different to what you normally see. 40v +/- on a phono pre-amp

I ran it for a week on my system on both vinyl and CD. It sounded fantastic. It wipes the floor with Technics amp that's on my avatar. I have a Paul Simon record on Vinyl that I also have now on CD. The Vinyl really did sound good, much better than I'd heard it before.

Perhaps the ultimate praise came from my wife. She sat down and said 'what's this? It sounds so clear and natural' She did not see the Sansui amp!

Cheers, SJM.

[stevehertz]

Good job. I restored a Sansui 9090 receiver a few years ago, the biggest challenge being misleading mistakes on the schematic diagram. Like you say, you'll go a long way to beating vintage Sansui amplifiers. Similarly, certain Pioneer, Yamaha, Luxman, Rotel and Technics amps and receivers. You can't beat lots of iron.